Tourists in Mexico City use masks to filter air pollution. PIc: Esparta Palma (Flickr CC)

On Thursday, authorities in Mexico City lifted an air pollution emergency alert that had extended into its fourth day. It was the first such alert in 11 years.

In order to cope with the dangerous air quality in the Mexican Federal District, over a million cars were ordered off the roads, while the public was given free access to the city’s public transport. Furthermore, residents were advised to limit their time outdoors.

Ozone levels in Mexico, DF were measured at almost two times acceptable limits.

Do temporary anti-pollution measures work?

There is some controversy surrounding temporary measures designed to combat air pollution and mitigate health risks.

In Shanghai, the advice to limit outdoor activity or “stay indoors” during days with particularly poor air quality has come under fire. Some researchers believe indoor pollution may be just as dangerous as its outside counterpart, at least in terms of particulate matter (PM2.5), volatile organic compounds (VOC), microbial organisms and CO2.

That said; there is no doubt that when industrial activity and road traffic are cut, the air gets better. Beijing’s “parade blue” is a perfect example.

Last September, in anticipation of a parade marking the 70th anniversary of the end of World War II, China stopped operations at hundreds of factories and cut car traffic in half in the capital. Beijing, now world-famous for its choking smog, had clear blue skies and boasted a healthy air quality index (AQI) of 17/500. However, the next day saw business return to normal and the AQI climbing to an “unhealthy” 160/500.

Beijing implemented similar restrictions to improve its air during the 2008 Olympic games.

PHOTO: Haze hangs over Mexico City as authorities ban more than 1 million cars from the roads https://t.co/bbUMezFYK6

The problem with temporary measures is that the pollution always comes back. For this reason, year-round measures to reduce road traffic have been tried in various territories around the world.

Mexico City already has a system of road space rationing called “Hoy No Circula,” in which vehicles with certain licence plate numbers are banned from operating on particular days. However, problems occur when wealthier residents get around the ban by purchasing another vehicle with a chosen license plate number or simply pay fines for driving on days when their cars are banned. Furthermore, older, more polluting cars are sometimes purchased as second vehicles for those less affluent drivers wishing to avoid the scheme.

Complications such as these highlight flaws such as unfairness and even the backfiring of Hoy No Circula, but they are details in the system’s implementation and not at all inherent in all road space rationing.

Alternatively, a congestion charge, such as what has been implemented in London, also allows those with wealth to circumvent inconvenience.

A robust and extensive public transport system — particularly one that runs on green energy or at least produces limited emissions — is key to reducing pollution in any large urban environment. Other measures include limiting coal use and the burning of organic material for cooking purposes — a major problem in the cities of the developing world. Together with these types of initiatives, a well-thought-out, year-round road space-rationing scheme could help to keep air quality within liveable levels, even in a megacity like Mexico, D.F.